Stabilization of photochromic copy

ABSTRACT

THE STABILIZATION OF COPY PRODUCED BY EXPOSURE OF A PHOTOCHROMIC LAYER FORMED OF HETEROPOLY ACIDS OF MOLYBDENUM AND TUNGSTE AND A REDUCING AGENT WHEREIN THE LAYER AFTER EXPOSURE IS WET WITH AN AQUEOUS ALKALINE SOLUTION OF AN ALKALINE MATERIAL.

United States Patent 3,623,866 STABILIZATION 0F PHOTOCHROMIC COPYAlexander Averbach, Skokie, Ill., assignor to A. B. Dick Company, Niles,Ill. No Drawing. Filed Nov. 1, 1968, Ser. No. 772,800 Int. Cl. G03c /24U.S. Cl. 96-48 4 Claims ABSTRACT OF THE DISCLOSURE The stabilization ofcopy produced by exposure of a photochromic layer formed of heteropolyacids of molybdenum and tungsten and a reducing agent wherein the layerafter exposure is wet with an aqueous alkaline solution of an alkalinematerial.

The sensitivity of such photochromic systems to light and depth ofshades developed has been found to depend, at least in part, on thenature of the reducing substance or substances employed in the system.

In my copending application Ser. No. 772,799 filed concurrentlyherewith, and entitled A Photochromic Element, description is made ofthe marked improvement in color intensity as well as stability of theimage formed upon exposure to ultra-violet light by the modification ofthe photochromic coating to include a phenylglycolic acid or derivativethereof.

It has been found, however, that such photochromic coatings, especiallywhen formulated to increase the sensitivity to light, as by theinclusion of such phenylglycolic acids or derivatives, are subject tothe development of background color, as by blackening or coloring of theotherwise white non-imaged portions of the sheet, when the imaged sheetis exposed to natural or artificial light for a short period of time.Blackening of the background occurs more rapidly when exposed to directsunlight or rays having a substantial amount of ultra-violet.

It is an object of this invention to provide a method for stabilizationof a photosensitive sheet of the type described against the developmentof background color in response to light exposure and it is a relatedobject to provide a method and means for stabilization of thephotochromic sheet whereby the non-imaged portions of the sheet resistdiscoloration during light exposure and otherwise remain white formaximizing the contrast between the developed image and the background.

Tungstic and molybdic heteropoly acids which may be used in the practiceof this invention are illustrated in Patent No. 2,981,622, which isincorporated hereby by reference, namely: silico-12-tungstic acid, H SiWO boro-12-tungstic acid, H BW O phospho-l2-tungstic acid, H PW Ophospho-9-tungstic acid, H PW O phospho-lZ-molybdic acid, H PMo Osilico-lZ-molybdic acid, H SiMo O phospho 12 tungstomolybdic acids,H3PW2MO3O40. The heteropoly acids may be used singly or in admixturewith each other. The term heteropoly acid is here used generically andincludes both free acids and their salts in which one or more of thehydrogen ions shown in the formulae above have been replaced by anothercation, usually an alkali metal ion. These salts are themselves acid insolution.

As reducing agents, use can be made of the agents described in theaforementioned issued patent and preferably dextrose, stannous chloride,hydroquinone, hydroquinone monoethyl ether, ascorbic acid, benzoin,desoxybenzoin, phenyl hydrazine, alpha bromophenyl acetic acid,gluconic, maleic, citric, adipic, lactic, tartaric and succinic acids,and more preferably phenylglycolic acid derivatives such as described inthe aforementioned copending application, including mandelic acid,benzilic acid,

ice

methyl mandelate, methenamine mandelate and benzylamine mandelate.

As binders for the coating, use is made of various film formers such aspolyvinyl butyral, polyvinylacetate, chlorinated rubber, polyvinylpyrrolidone, polyacrylamide, gelatin, polyvinyl alcohol and vegetablegums.

It has been found that the discoloration of the background or non-imagedportions of the sheet and particularly the blackening that isexperienced with the photochromic systems, the light sensitivity ofwhich has been increased by the presence of phenylglycolic acids andderivatives, can be overcome by stabilization of the background bypassing the paper, after exposure to produce copy, through an aqueousalkaline bath. A suitable aqueous alkaline bath can be formulated bysolution of an alkaline base material in aqueous medium in which thealkaline base material may be represented by the following:

(1) salts of a weak acid and a strong base (2) ammonium hydroxide (3) aquaternary ammonium compound The above may be illustrated by thefollowing examples:

EXAMPLE 1 5-15% and preferably 810% by weight of sodium or potassiumcarbonate, with the remainder water.

EXAMPLE 2 9-27% by weight of ammonium hydroxide, with the remainderWater.

EXAMPLE 3 5-15% and preferably about 10% by weight of tetraethylammoniumhydroxide, with the remainder Water.

Photochromic coatings which are suitable in the practice of thisinvention are described in the aforementioned issued US. patents and inthe aforementioned copending application.

Treatment of the photochromic coating after development with thealkaline solution to stabilize the copy against blackening anddiscoloration in the non-imaged portions can be made by Wetting the copyafter exposure with one of the solutions of Examples 1 to 3, by swabbingthe surface after exposure with a cotton pad wet with the solution.

In another example, in which a base sheet has a photochromic coatingformulated of 9.5% by Weight polyvinyl alcohol, 14% by Weightphosphotungstic acid and 7% by weight mandelic acid, exposure was madeto ultraviolet through a negative to produce copy. One-half of theexposed sheet was swabbed with a 2628% by Weight ammonium solution inwater and dried. The dried imaged sheet was thereafter exposed to lightfor one-half hour. The swabbed portions did not blacken or discolorwhereas the portions that were not swabbed turned grey in thebackground.

It was somewhat surprising that corresponding stabilization effects areincapable of being secured by treatment of the surface with dry ammoniumgas, organic amines, or solutions of salts of strong inorganic acids andbases. It was also found that strong bases, such as sodium hydroxide,even in such dilute solutions as 1-2% by weight, adversely affect theimage.

Treatment in accordance with the practice of this invention results incopies which can be exposed for eight to ten weeks in ambient lightwithout appreciable change in background.

It will be apparent from the foregoing that I have provided a simple andefiicient means for stabilization of photochromic copies againstdiscoloration in the background in response to light exposure wherebymore permanent copy of good copy quality can be achieved by suchphotochromic techniques.

It will be understood that changes may be made in the details offormulation and operation Without departing from the spirit of theinvention, especially as defined in the following claims.

I claim:

1. The method of producing a stabilized photochromic copy comprisingexposing a copy sheet having a photochromic coating formed of aheterpoly acid of molybdenum and tungsten and a reducing agent for thedevelopment of color change in the exposed portions, wetting the surfaceof the photochromic coating with an aqueous alkaline solution consistingessentially of an alkaline material selected from the group consistingof the salt of a weak acid and a strong base, ammonium hydroxide and aquaternary ammonium compound and Water after exposure to stabilize thenon-imaged portions of the coating against discoloration.

2. The method as claimed in claim 1 in which the reducing agent is aphenyl glycolic acid or derivatives thereof.

3. The method as claimed in claim 1 in which the photochrornic compoundis selected from the group consist ing of tungstic and molybdic acid andcomplex formed thereof with phosphoric, boric or silicic acid.

4. The method as claimed in claim 1 in which the alkaline material isselected from the group consisting of a salt of a Weak acid and a strongbase present in solution in an amount Within the range of 515% byweight, 927% by weight ammonium hydroxide, and a quaternary ammoniumcompound present in solution in an amount within the range of 515% byweight.

1 References Cited UNITED STATES PATENTS 1,934,451 11/1933 Sheppard etal. 957 2,427,443 9/1947 Cochran 957 2,855,300 10/1958 Chalkley 96-482,895,892 7/1959 Chalkley 204157 2,915,392 12/1959 Pedersen 96272,981,622 4/1961 Chalkley 9648 3,169,064 2/1965 Levy 9627 3,236,6512/1966 Marks et al 96-88 3,285,746 11/1966 Marks et al 9690 3,293,03712/1966 Chopoorian 969O 3,322,542 5/1967 Fisherullman 9690 3,322,5525/1967 Roberts et al. 23-50 X 3,361,706 1/1968 Meriwether et al. 9690 PCFOREIGN PATENTS 658,273 6/1965 Belgium 9690 PC 727,559 2/1966 Canada969O PC 736,192 11/1932 France 96-90 OTHER REFERENCES Meriweather,Journal of the American Chem. Soc, vol. 87, No. 20, Oct. 20, 1965, p.44414454.

WHJLIAM D. MARTIN, Primary Examiner B. D. PIANALTO, Assistant ExaminerUS Cl, X.R, 96-90; 252300 3, 623, 866 Dated November 30, 1971 Patent No.

In ent Alexander Averbach It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 1, after line 17, insert This invention relates to a photographicprocess for producing images by photosensitive inorganic photochromic orpho totropic substances which acquire a color change upon exposure tolight.

Photochromic compounds may be subdivided into organic substancesrepresented by spyrans and inorganic substances represented byheteropoly acids of tungsten and molybdenum,

such as phospho-tungstic and phospho-molybdic acids.

This invention has to do ith the latter group of inorganic photochromiccompounds and their utilization in the production of images by reason oftheir longer reversion periods,

as is or in stabilized form, whereby they are better adapted forprint-out and other copy processes. The sensitivity of such inorganic heteropoly acids of tungsten and molybdenum are greatly increased whenemployed in the form of tungstic and molybdic complexes with phosphoric,boric or silicic acids,

including mixtures of such tungstic and molybdic complexes ORMPO-1050'1O69) uscomm-oc 60376-969 U S. GCVEHNMENT PRNTING OFFWCE 59690-365-33 Patent 3,623,866 Dated November 30 1971 Alexander Averbach PAGE2 Inventor(s) It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Photochromic cohp nnds an d coniplexes thereof and their utilization inhotosensitive products are well known to the art. Reference may be madeto the following United States patents: No. l,93 #51; No. 2, ;27, +u3;No. 2,895,892; No.

2,981,622; No. 3,169,064; No. 3,236,651; No. 3,285,746; No.

3,293,037 and No. 3,322,552. Such photochromic processes andcompositions have not experienced widespread usage in the copy fieldbecause of the inability to obtain colors and hues sufficiently deep fora desired color contrast. Furthermore, systems of the type heretoforeemployed have been characterized by too rapid reversion after removal ofthe ultra-violet light whereby the image remains for only a relativelyshort period of time:

Signed and sealed this 20th day of June 1972.

(SEAL) Attest:

EDWARD M. FLETCHER,JR. ROBERT GOTTSCHALK Attestlng Officer Commissionerof Patents OHM [10459) USCOMM-DC 50376-P69 U75, GOVERNMENY PRINTINGOFFCE I959 0-355-334

